Such devices are previously known and are applied in different areas of technology. It is thus previously known to measure the mass concentration in a paper pulp suspension by measuring a resisting torque when rotating a sensor in the suspension to be investigated. The measured torque is based on the force exerted by the rotating body for separating fibres in the suspension. The force and thus the torque thus depends of the quantity of fibres in the suspension. It is further previously known to avoid stuff box friction etc. by leading the rotating shaft which transmits the torque through a hollow shaft and use a sensor functioning according to the principle of equilibrium of forces. This principle is based on the fact that because of an exerted torque, a displacement will occur between the transmitting shaft and the hollow shaft which displacement may be detected. Known measuring methods suffer however from disadvantages with respect to errors in measurement, i. a. due to the connection between the sensor shaft and the hollow shaft. Also in other respects the known measuring methods are unreliable.
It should be noted that the consequences of measurement errors can be substantial. As an example in paper manufacture, if a paper pulp suspension is to be investigated, the economically acceptable concentration interval ranges in a variation of dry contents of only about one percent. Too low concentration may lead to breakage of the resulting paper web, too high concentration results in waste of fibre and too thick paper.